Joint implants, also referred to as joint prostheses, joint prosthetic implants, joint replacements, or prosthetic joints, are long-term surgically implantable devices that are used to partially or totally replace diseased or damaged joints, such as a hip, a knee a shoulder, an ankle, or an elbow joint, within the musculoskeletal system of a human or an animal. Since their first introduction into clinical practice in the 1960s, joint implants improved the quality of life of many patients. Both artificial hip joints and artificial shoulder joints are generally ball and socket joints, designed to match as closely as possible the function of the natural joint. Generally, the artificial socket is implanted in one bone, and the artificial ball articulates in the socket. A stem structure attached to the ball is implanted in another of the patient's bones, securing the ball in position.
The ball and socket joint of the human hip unites the femur to the pelvis, wherein the ball-shaped head of the femur is positioned within a socket-shaped acetabulum of the pelvis. The head of the femur or ball fits into the acetabulum, forming a joint which allows the leg to move forward, backward and sideways in a wide range. The acetabulum is lined with cartilage, which cushions the bones and allows the joint to rotate smoothly and with minimal friction. An envelope of tough ligaments connects the pelvis and femur, covering the joint and stabilizing it. Cartilage also makes the joint strong enough to support the weight of the upper body and resilient enough to absorb the impact of exercise and activity. A healthy hip allows the leg to move freely within its range of motion, while supporting the upper body and absorbing the impact that accompanies certain activities.
However, various degenerative diseases and injuries may require replacement of all or a portion of a hip using synthetic materials. Prosthetic components are generally made from metals, ceramics, or plastics or combinations of them.
Total hip arthroplasty and hemi-arthroplasty are two procedures well known within the medical industry for replacing all or part of a patient's hip and have enabled hundreds of thousands of people to live fuller, more active lives. A total hip arthroplasty replaces both the femoral component and the acetabular surface of the joint, so that both a femoral prosthesis and an acetabular prosthesis are required. A hemi-arthroplasty may replace either the femoral component or the acetabular surface of the joint. The purpose of hip replacement surgery is to remove the damaged and worn parts of the hip and replace them with artificial parts, called prostheses, which will help make the hip strong, stable and flexible again.
In hip replacement surgery, commonly referred to as total hip arthroplasty, a patient's natural hip is replaced by two main components, a stem member which takes the place of the femoral head, and an acetabular cup member which takes the place of the acetabular socket.
A conventional acetabular cup member may include a cup, a cup and a liner, or in some cases only a liner, all of which may be formed in various shapes and sizes. Generally, a metal cup and a polymeric liner are used. However, the liner may be made of a variety of materials, including polyethylene, ultra high molecular weight polyethylene and ceramic materials. The cup is usually of generally hemispherical shape and features an outer, convex surface and an inner, concave surface that is adapted to receive a cup liner. The liner fits inside the cup and has a convex and concave surface. The cup liner is the bearing element in the acetabular component assembly. The convex surface of the liner corresponds to the inner concave surface of the cup or acetabulum, and the liner concave surface receives the head of a femoral component. An acetabular cup may include a highly polished inner surface in order to decrease wear.
The stem and ball portion of the prosthesis may be a femoral prosthesis that generally includes a spherical or near-spherical head attached to an elongate stem with a neck connecting the head and stem. In use, the elongate stem is located in the intramedullary canal of the femur and the spherical or near-spherical head articulates relative to the acetabular component. Femoral prostheses used in total hip arthroplasty procedures may or may not differ from an endoprosthesis used in a hemi-arthroplasty. The femoral head of each type prosthesis is generally a standard size and shape. Various cups, liners, shells, stems and other components may be provided in each type arthroplasty to form modular prostheses to restore function of the hip joint.
During a total hip replacement, the surgeon will take a number of measurements to ensure proper prosthesis selection, limb length and hip rotation. After making the incision, the surgeon works between the large hip muscles to gain access to the joint. The femur is pushed out of the socket, exposing the joint cavity. The deteriorated femoral head is removed. In order to install the acetabular cup, the surgeon must prepare the bone by reaming the acetabular socket to create a surface suitable for accepting a cup, which may be held in place by bone cement, an interference or press fit, or it may have a porous outer surface suitable for bony ingrowth. The new acetabular shell is implanted securely within the prepared hemispherical socket. The plastic inner portion of the implant is placed within the metal shell and fixed into place.
Next, the femur is prepared to receive the stem. The hollow center portion of the bone is cleaned and enlarged, creating a cavity that matches the shape of the implant stem. The top end of the femur is planed and smoothed so the stem can be inserted flush with the bone surface. If the ball is a separate piece, the proper size is selected and attached. Finally, the ball is seated within the cup so the joint is properly aligned and the incision is closed.
The ball and socket joint of the human shoulder is prepared using a procedure similar to that described above. During a shoulder replacement operation, at least a portion of the proximal section of the humeral shaft is replaced by a metal prosthesis. This prosthesis generally consists of two parts: a stem that is mounted into the medullary canal of the humerus, and a head component connected in some manner to the stem. The head component replaces the bearing surface of the humerus and articulates within the glenoid cavity of the scapula to allow movement of the shoulder.
An arthritic humeral head (ball of the joint) may be removed and replaced with a humeral prosthesis. If the glenoid socket is unaffected, a hemiarthroplasty may be performed (which means that only the ball is replaced). The humeral component is made of metal and is usually press fit, but sometimes cemented, into the shaft of the bone of the humerus.
If the glenoid is affected, but conditions do not favor the insertion of a glenoid component, a non-prosthetic glenoid arthroplasty may be performed along with a humeral hemiarthroplasty. In this procedure, the glenoid shape and orientation are corrected, but a glenoid prosthesis is not inserted. The socket is reshaped by reaming and the prosthetic ball of the humeral component articulates with the reshaped bony socket of the glenoid.
In a total shoulder joint replacement, the glenoid bone is shaped by reaming and oriented, and then covered with a glenoid component. A small amount of bone cement is commonly used to hold the artificial glenoid socket in place.
During joint replacement surgery, such as the procedures described above, typically, a rather large incision is required to allow the surgeon adequate access to the joint in order to use the instruments needed to prepare the bones, such as femur and acetabulum, to receive the prosthetic members, and to install the prosthesis itself.
During recent years, an effort has been made to reduce the size of the suture needed to implant joint prostheses, through so-called “minimally invasive” surgery. The benefits of such surgery can be significant, given the smaller incision and less intrusive nature of the procedure, shortening both surgical time and recovery time. It has been advantageous to modify traditional implants and instruments to make them particularly suitable for use in minimally invasive surgical procedures.
Reamers are devices for reaming bone tissue during reshaping of bone, specifically, in preparation for installation of joint prostheses. Reamers may also be used in connection with various trauma situations, and so forth. Reamers may have a spherical or other shaped dome if the cavity being shaped is spherical or concave, or may have an elongated shape if the cavity being shaped is a canal. An acetabular reamer is a device for removing bone tissue from an acetabular socket of an animal or a human in preparation for installation of an acetabular component of a hip prosthesis. Acetabular reamers typically have a generally hemispherical or convex dome shape with a plurality of holes and raised edges serving as cutting teeth for removing bone and cartilage from the natural socket to form a relatively smooth, concave shaped surface that generally conforms to the hemispherical external surface of an acetabular cup. The reamers may be attached to a handle, which, when connected to the reamer, allows rotation around the access of the reamer dome. Attachments of the handle is through a bracket inside the reamer dome, or by press-fitting or welding into the back side of the reamer dome. The reamers typically used for preparation of glenoid sockets are similar to those described above for use with the preparation of a hip socket for receiving an acetabular cup. During use, the reamer is rotated either by a hand-operated or power tool, so that the cutting teeth gradually remove cartilage and bone in small pieces, which pass through the holes and are, for the most part, contained within the concave interior portion of the reamer dome and are removed upon withdrawal of the reamer from the wound. It is important to remove as many of the bone chips and other particles as possible prior to completing the surgery, since they can cause problems for the patient if allowed to remain in the joint cavity. The reamers can be used in conjunction with image guided navigational systems, or other systems for precision guiding of the reamers.
For minimally invasive hip arthroplasty, it has been discovered that traditional acetabular reamers are difficult to insert and operate through a smaller incision, due to their bulk and the existence of protruding cutting teeth on their entire outer surface. The rather large diameter of a conventional acetabular reamer damages the flesh, muscle, and other soft tissue during insertion and withdrawal of the reamer through the minimally invasive incision.
One known method to minimize the incision necessary for introduction and withdrawal of a reamer is to simply remove two opposing sides of the reamer, resulting in a reamer dome having a lower profile by virtue of its reduced diameter in one direction. A recognized disadvantage of this design is that the open sides allow chips and smaller particles of reamed bone to migrate from the interior cavity of the reamer into the wound, and must be carefully removed, thereby adding both time and risk to the overall surgical procedure.
Therefore, there is a current unrealized need for an improved reamer adapted for use in minimally invasive procedures. There is a particular unrealized need for an improved acetabular reamer adapted for use in minimally invasive procedures. An improved reamer is desired that is adapted for introduction and operation through a smaller surgical incision than conventionally available reamers. Also needed is an improved reamer that would minimally damage the flesh, muscle, and other soft tissues during insertion, operation, and withdrawal. There is a current unrealized need for an improved reamer adapted for use in minimally invasive procedures, which reliably captures the reamed bone particles, at least as well as a conventional reamer, in order to prevent the particles from falling into the wound.
In general, reamers are needed that are easy to use and manufacture, minimize tissue damage, simplify surgical procedures, are versatile, allow for faster healing with fewer complications, require less post-surgical immobilization, and are less costly to produce and operate.